Okay Bert, thanks for that extra information. It is very helpful. From my experience, you have possibly have two different things going on here, both of which are serious. Staying at the bottom of the cage, keeping feathers fluffed, are symptoms of an ill bird. Actually a very ill bird as they do not allow us to see symptoms until they have become too weak to keep up the pretense. the pulling and plucking at feathers under the wing is one of the symptoms of either Giardia or possibly Clamydia. Either of those is going to need a definite diagnosis from a well qualified avian vet so proper treatment can get started immediately. Either way it will likely involve a test of one of her droppings, a 10-14 day course of oral antibiotics and probably a topical cream to put on the plucked areas because either of these can make her quite itchy. And of course that becomes "bitchy" when you try to handle her. You should not be handling her at all right now. Two reasons for that. An ill bird should be kept at the lowest possible stress levels until they can get to the vet. The other reason has to do with the other behavior you described; apparent straining. If in fact you do have a female bird, it's very possible she is egg bound. That is a life threatening emergency. Handling her could rupture the egg inside of her and that is even more of an emergency. If she is egg bound, you should be able to see a bulge in her abdomen, near the vent area. If it continues, it will start to press on nerves and blood supply to her legs and feet. If she has not had enough calcium in her diet, (unending supply of cuttlebone for one), she is not able to form solid shells for her eggs. Hence, they are too soft and she cannot pass them. She is also not on a healthy diet, based on what you said she is eating. She should have far less sunflower seeds, not more. She should not have more than 4-5 sunflower seeds per day. They are very fattening and will lead to liver disease. She should also be getting a good brand of pellets and plenty of veggies, pastas, brown rice, hard boiled or scrambled eggs and many other items. Leaves of turnip greens and Kale are a good source of calcium. I'm going to give you some links to proper Cockatiel diet. You should start offering her these extra items right away and it is critical that she have plenty of cuttlebone. However, the diet is a long term concern. Your immediate concern is the infection and the possibility of being eggbound. Those two things need to be acted on immediately. In case you do not have a good avian vet, I'll also give you links to assist in locating one.
Life After Weaning - Your Companion Bird and You
Parrot C&onsultant
Published author, free lance bird behaviorist, adviser to the parrots at Sarasota Jungle Gardens.